Travis CI: Run flake on Python 2.7 and 3.7 (pytorch#9953)

Summary:
Flake8 will produce different results on Python 2 and 3.  Python 3.7 has __async__ as a reserved word pytorch#4999.
Pull Request resolved: pytorch#9953

Differential Revision: D9035415

Pulled By: soumith

fbshipit-source-id: 8a46e028a2e20a7e3f6d90137020268d65a7cc64

.travis.yml

@@ -16,6 +16,12 @@ matrix:
         python: "2.7"
         install: pip install flake8
         script: flake8
+      - env: LINT_CHECK
+        python: "3.7"
+        dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
+        sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
+        install: pip install flake8
+        script: flake8
       - env: MYPY_TYPE_CHECK
         python: "3.6"
         install: pip install mypy mypy-extensions